<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .father .son .sun {
      color：red;
      font-size: 20px;
    }
  </style>
</head>
<body>
  <div class="father">
    <div class="son">
      <div class="sun">孙子</div>
    </div>
  </div>
</body>
</html>